The invention relates to a method and a device for preventing uncontrolled spread of powder in a printing machine. The invention relates in particular to a method in which sheets printed in a sheet-fed printing machine are conveyed through a sheet delivery system above a sheet deposit pile by a sheet conveying device and, in the process, are dusted with a powder, and wherein uncontrolled spreading of excess powder is prevented, and to a device for preventing uncontrolled spreading of powder in a housing of a sheet delivery system of a sheet-fed printing machine, the housing surrounding a sheet conveying device for transporting printed sheets over a sheet deposit pile, and a dusting device for dusting the printed sheets with a powder. Furthermore, the invention relates to a sheet delivery system for a sheet-fed printing machine and to a sheet-fed printing machine having such a device for preventing the uncontrolled spreading of powder.
During sheet-fed offset printing in larger sheet-fed offset printing machines, the paper sheets printed on one or both sides, after passing through the printing units of the printing machine, are usually picked up by a chain conveyer of the paper or sheet delivery system, i.e., a chain conveyer having two parallel chains connected by delivery gripper systems or sheet grippers and, within a housing that encloses the sheet delivery system and the printing units of the printing machine, are transported to a location over a sheet pile, whereon they are subsequently deposited above one another. Because the printing ink has not yet completely dried when the sheets are piled above one another, in most of the sheet-fed offset printing machines on the market, at least one surface of each sheet, before the sheet is deposited on the pile, is dusted with a powder which sticks to the printed parts of the paper sheets and prevents set-off, i.e., a sticking together of the paper sheets in the sheet pile. The powder that is used is usually finely powdered potato starch or other powdery starch products. In order to apply the powder, use is made of a so-called spray powder device or any other dusting device which, as a rule, is arranged in an intermediate space between the forward-running conveyor strand transporting the sheets and the returning empty strand of the chain conveyer, and dusts, with powder from above, the sheets which are carried by the lower conveyor strand of the chain conveyer and transported past the powder device.
High-speed sheet-fed offset printing machines for printing large editions, such as the sheet-fed offset printing machines sold by Heidelberger Druckmaschinen AG under the designation Heidelberg Speedmaster, are usually provided with a high-pile sheet delivery system, in order to be able to stack or pile up more sheets. In such sheet delivery systems, the conveying strand of the chain conveyer initially extends obliquely upwardly and is then deflected into the horizontal, in order to align the printed sheets horizontally before releasing them over the sheet deposit pile. The dusting device is disposed thereat usually just before the upper end of the oblique portion of the conveying strand, in the interspace between the latter and the empty strand of the chain conveyor.
During the operation of sheet-fed offset printing machines of this type, it has been determined that considerably more powder is consumed than is actually necessary for dusting the paper sheets. At the same time, the ambient air around the sheet delivery system has a considerably increased dust content, caused by powder particles, while deposits of powder form to an increased extent on movable and immovable parts in the interior of the housing of the sheet delivery system. The high dust content in the ambient air which, to some extent, can be referred to as a virtual powder mist, is found to be extremely unpleasant by the operating personnel, while deposits of powder in the interior of the housing, in particular on movable parts, lead to increased wear and relatively long downtimes of the machine, due to a requirement for more frequent cleaning and maintenance work.
During investigations and testing by the applicant to clarify the causes of the dust loading caused by powder in the ambient air, as well as the powder deposits in the housing of the sheet delivery system, it has been established that air turbulence occurs to a great extent in the sheet delivery systems of conventional offset printing machines. This air turbulence is caused, inter alia, by the fact that, in each case, underneath and above the conveying strand and the empty strand of the sheet delivery, air is entrained in the direction of motion of the respective strand as a result of the suction action of the moving sheet grippers and chains, by the fact that printed sheets entrained by the conveying strand flutter or, when being deflected from the oblique into the horizontal position, flip upwardly at the rear or trailing edge thereof (a so-called whipping or flapping effect), and by the fact that air is blown from below towards the sheet from the so-called sheet guiding plate, in order to guide the sheet over the guide plate without contact, but some of this air flows upwardly between the adjacent sheets and laterally past the respective sheet.
Due to this air turbulence, a great part of the powder emerging from the powder dusting device is swirled about and entrained by the air so that it does not pass onto the surface of the printed material directly underneath the powder dusting device, as desired. Instead, the powder is distributed over the entire interior of the sheet delivery system by the airflows, in particular those along the two strands of the sheet delivery. This results not only in increased wear of moving parts and longer downtimes for cleaning the sheet delivery system, but also to irritation or disturbance of the operating personnel, because the swirled powder also escapes into the environment through all the openings in the sheet delivery system, in particular through the sheet outlet opening and air outlet gratings on the top of the housing.
In order to reduce the high dust loading of the ambient air and to eliminate further problems caused by excessive powder, in U.S. Pat. No. 5,265,536, a suction hood arranged above the sheet pile and a sheet outlet opening is proposed, which serves for extracting or sucking powder-laden air upwardly through the empty strand, in order then to clean the air. Some of the powder-laden air, however, is entrained by the suction of the empty strand as it passes therethrough and flows along the empty strand in the direction of the printing machine.
In addition, the published German Patent Document DE 42 07 118 A1 discloses a sheet delivery system in a sheet-fed printing machine, having a housing wherein, in the vicinity of the powder dusting device above the delivery strand, devices are provided for extracting or sucking excessive powder and, therefore, for preventing the uncontrolled spreading thereof. Devices of this type immediately extract or suck up a large portion of the applied powder again, but this often has the result that the operating personnel increase the supply of powder, and in this way even more powder is consumed. On the other hand, the problem of powder spread due to air suction in the interior of the housing is not solved.
In addition, the German Published Prosecuted Patent Application (DE-AS) 2 148 757 discloses a sheet delivery system in a printing machine wherein, along an upper horizontal portion of the empty strand of a sheet conveyer, a cleaning device is provided, which includes a housing, a number of brushes arranged in the housing and an extraction or sucking device for extracting or sucking away the powder cleaned off the chains and gripper systems by the brushes. However, in this published German patent application, neither the problem of the spread of powder due to the powder-laden air entrained by the suction of the empty strand was detected, nor is the described cleaning device suitable for preventing the transport of powder-laden air in the direction of the printing machine, because there are wide, open flow cross sections both between the housing of the cleaning device and the adjacent walls of the sheet delivery system and also within the housing on both sides of the brushes, and these permit the passage of powder-laden air along the path of movement of the empty strand.
Starting from this state of the art, it is an object of the invention to provide a method and a device of the general type mentioned at the introduction hereto, which can be assembled after leaving the factory and/or at the factory, which permits a reduction in the dust loading in the environment of the printing machine during the printing operation and which largely prevents the formation of powder deposits within the housing.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method for conveying sheets, which have been printed in a printing machine, through a sheet delivery system above a sheet deposit pile by a sheet conveying device and, at the same time, dusting the sheets with a powder, and preventing uncontrolled spread of excess powder, which comprises interrupting a powder-laden air flow extending in a direction towards the printing machine along an empty strand of the sheet conveying device in a housing of the sheet delivery system at least at one interruption location between a free end of the sheet delivery system and the printing machine, so that transport of entrained powder in a direction towards the printing machine is prevented.
In accordance with another mode, the method of the invention includes substantially completely blocking a flow path between an upper wall or cover of the housing, side walls of the housing and a device that is disposed underneath the empty strand and above a conveying strand of the sheet conveying device and that serves as a dividing wall, so as to prevent powder-laden air from passing by to the printing machine.
In accordance with a further mode, the method includes interrupting the airflow in an upper horizontal portion of the sheet delivery system.
In accordance with an added mode, the method includes interrupting the airflow above a drive shaft of a lifting mechanism for the sheet deposit pile.
In accordance with an additional mode, the method includes feeding air not laden with powder into the housing in a manner that the air flows through the empty strand transversely to the direction of motion thereof, so as to interrupt the airflow.
In accordance with yet another mode, the method includes feeding the air into the housing from above.
In accordance with yet a further mode, the method includes feeding-in the air in a direction of motion of the empty strand and downline of a brush device at least partly blocking the flow path.
In accordance with yet an added mode, the method includes deflecting at least some of the air that is fed-in, in the direction towards the printing machine.
In accordance with yet an additional mode, the method includes guiding at least some of the deflected air in the direction towards the printing machine over and past a dusting device for dusting the sheets with powder.
In accordance with still another mode, the method includes extracting powder-laden air in the vicinity of the interruption location, so as to interrupt the airflow.
In accordance with still a further mode, the method includes extracting at least some of the air fed into the housing, after the air has passed through the empty strand.
In accordance with still an added mode, the method includes extracting the air nearly opposite the location at which it is fed into the housing.
In accordance with still an additional mode, the method includes extracting the powder-laden air in the vicinity of a brush device that at least partly blocks the flow path.
In accordance with another mode, the method includes, in at least one of the region of the interruption location and a location downline thereof in the direction of motion of the empty strand, sucking air into the housing from the outside through at least one air opening.
In accordance with a further mode, the method includes interrupting the airflow with a brush device at least nearly completely blocking the flow path.
In accordance with a second aspect of the invention, there is provided a device for preventing uncontrolled spread of powder in a housing of a sheet delivery system of a sheet-fed printing machine, the housing surrounding a sheet conveying device for transporting printed sheets over a sheet deposit pile and a dusting device for dusting the printed sheets with a powder, comprising devices for interrupting a powder-laden air flow extending in a direction towards the printing machine along an empty strand of the sheet conveying device, the flow-interrupting devices being mountable at at least one interruption location between a free end of the sheet delivery system and the printing machine for preventing transport of entrained powder in the direction towards the printing machine.
In accordance with another feature of the invention, the flow-interrupting devices at least nearly completely block, by at least one mechanical and pneumatic device, a flow path between an upper wall or covering of the housing, side walls of the housing and a device that is disposed underneath the empty strand and above a conveying strand of the sheet conveying device and serves as a dividing wall, for preventing powder-laden air from passing by to the printing machine.
In accordance with a further feature of the invention, the flow-interrupting devices include at least one air feed device for feeding air not laden with powder into the housing, so that the air flows through the empty strand transversely to a direction of motion of the latter.
In accordance with an added feature of the invention, the flow-interruption devices include at least one extraction device for extracting powder-laden air in the region of the interruption location.
In accordance with an additional feature of the invention, the extraction device is disposed at least nearly opposite an air feed device for extracting at least some of the air fed into the housing by the air feed device after the air has passed through the empty strand.
In accordance with yet another feature of the invention, the extraction device and the air feed device are arranged above one another.
In accordance with yet a further feature of the invention, the extraction device is disposed underneath the empty strand and has at least one suction opening at the top thereof, facing towards the empty strand.
In accordance with yet an added feature of the invention, in the direction of motion of the empty strand, the air feed device is disposed downline from a brush device which at least partly blocks the flow path.
In accordance with yet an additional feature of the invention, the extraction device is disposed in vicinity of a brush device which at least partly blocks the flow path.
In accordance with still another feature of the invention, the flow-interrupting devices include a brush device at least nearly completely blocking the flow path.
In accordance with still a further feature of the invention, the brush device includes at least two brushes having bristles which overlap or touch in a path of movement of the empty strand.
In accordance with still an added feature of the invention, in a region of the interruption location and/or a location downline therefrom in a direction of motion of the empty strand, at least one air opening is located for permitting air not laden with powder to be sucked into the interior of the housing.
In accordance with still an additional feature of the invention, the air opening is located at the top of the housing, above the empty strand.
In accordance with another feature of the invention, in a direction of motion of a conveying strand of the sheet conveying device, at least some of the flow-interrupting devices are mountable or mounted downline of the dusting device.
In accordance with a further feature of the invention, at least some of the flow-interrupting devices are mountable or mounted in an upper horizontal portion of the sheet delivery system.
In accordance with an added feature of the invention, at least some of the flow-interrupting devices are mountable or mounted above a blower or an air-nozzle device for blowing the printed sheets downwardly onto the sheet deposit pile.
In accordance with an additional feature of the invention, in a direction of motion of a conveying strand of the sheet conveying device, at least some of the flow-interrupting devices are mountable or mounted downline of a location at which the conveying strand is deflected into the horizontal.
In accordance with yet another feature of the invention, at least some of the flow-interrupting devices are mountable or mounted at least above or upline and downline of a drive shaft belonging to a lifting mechanism for the sheet deposit pile and passing through an intermediate space.
In accordance with yet a further feature of the invention, the prevention device includes a deflection device mountable or mounted underneath the empty strand for deflecting at least some air fed-in in a direction towards the printing machine.
In accordance with a third aspect of the invention, there is provided a sheet delivery system for a sheet-fed printing machine, having a housing, a conveying device arranged within the housing for transporting printed sheets to a sheet deposit pile, and a dusting device for dusting the printed sheets with a powder, comprising a device for preventing uncontrolled spread of powder in the housing of the sheet delivery system, including devices for interrupting a powder-laden air flow extending in a direction towards the printing machine along an empty strand of the sheet conveying device, the flow-interrupting devices being mountable or mounted at at least one interruption location between a free end of the sheet delivery system and the printing machine for preventing transport of entrained powder in the direction towards the printing machine.
In accordance with a fourth aspect of the invention, there is provided a sheet-fed printing machine in combination with a sheet delivery system having a housing, a conveying device arranged within the housing for transporting printed sheets to a sheet deposit pile, and a dusting device for dusting the printed sheets with a powder, comprising a device for preventing uncontrolled spread of powder in the housing of the sheet delivery system, including devices for interrupting a powder-laden air flow extending in a direction towards the printing machine along an empty strand of the sheet conveying device, the flow-interrupting devices being mountable or mounted at at least one interruption location between a free end of the sheet delivery system and the printing machine for preventing transport of entrained powder in the direction towards the printing machine.
In accordance with a concomitant feature of the invention, the sheet-fed printing machine includes devices for cleaning the air that has been fed-in and/or extracted.
Thus, an air flow loaded with excess powder, extending in the direction of the printing machine along an empty strand of the sheet conveying device in the housing of the sheet delivery system, as a result of previously passing the dusting device by, is interrupted at at least one interruption location between the free end of the sheet delivery system and the printing machine, in order to prevent the transport of entrained powder in the direction of the printing machine.
A preferred configuration of the invention therefore provides for each flow path along the movement path of the empty strand, through which powder-laden air can flow from the free end of the sheet delivery system towards the printing machine to be blocked at at least one location between the free end of the delivery and the printing machine, in order in this way to prevent the formation of an air flow extending in the direction of the printing machine and, therefore, to prevent the transport of powder in this air flow towards the printing machine. For this purpose, it is necessary, at the interruption location, to essentially completely block the entire free flow cross section between an upper wall of the housing, the two side walls of the housing and a dividing wall or the like arranged in the intermediate space between the empty strand and the conveying strand, so that the passage of powder-laden air through this section of the flow path is virtually completely suppressed.
According to a further preferred configuration of the invention, the air flow is interrupted as close as possible to the free end of the sheet delivery system, i.e., expediently in an upper horizontal portion of the sheet delivery system and preferably above a drive shaft of a lifting mechanism for the sheet deposit pile, whereat there is sufficient space for the necessary devices. Because the drive shaft rotates very slowly, it can additionally be used well as support or carrier for some of the devices.
The interruption to the airflow can be carried out using pneumatic and/or mechanical devices, a combination being preferred. In the case of interruption with a pneumatic device, air is preferably extracted from the housing at the interruption location, and/or a stream of clean air, i.e., air not laden with powder, is fed into the housing of the sheet delivery system, both in such a way that the flow path is blocked by an air curtain or air flow, which passes through the empty strand and is sufficiently broad and deep to prevent the passage of powder-laden air between the upper wall of the housing, the two side walls of the housing and the dividing wall or the like arranged between the empty strand and the conveying strand.
To this end, the devices serving to interrupt the air flow preferably comprise an air feed device, which conducts air extracted from the environment and not laden with powder into the housing in such a way that it flows vertically through the empty strand, transversely with respect to the direction of motion of the latter, and/or an air extraction device, which extracts air from the flow path, transversely with respect to the direction of motion of the empty strand, preferably opposite the air feed device, in order to extract at least some of the introduced air, together with the powder that has showered off the chains and sheet grippers of the conveying device, after it has passed through the flow path, and expediently to clean it of entrained powder in a separator connected downline.
The air fed-in is preferably led into the housing from above and extracted underneath the empty strand, in order to carry away powder that has showered off in the direction of gravity. However, a converse arrangement is likewise conceivable.
In order to ensure a formation of the air curtain which is disturbed as little as possible, a number of brushes are preferably provided upline of the air feed device in the direction of motion of the empty strand, the brushes extending over the entire width of the existing flow path and the bristles thereof projecting from above and below into the movement path of the empty strand and touching or overlapping thereat, in order in this manner to block the flow path over the entire height thereof as well and between the individual sheet grippers and, at the same time, to clean the sheet grippers and chains of adhering powder.
The cleaning of the chains and sheet grippers is likewise of importance for preventing the spread of powder, because powder entrained by the chain conveyer and later released by vibration or the-like is deposited in the housing of the sheet delivery system and, particularly in the vicinity of the last printing station of the printing machine, can lead to increased wear of sensitive components and to poor print quality.
According to a particularly preferred configuration of the invention, downline of a pair of brushes, air from the environment or atmosphere is blown from above into the housing of the sheet delivery system through the empty strand, and at least some of the air blown in is extracted again underneath the empty strand, so that powder-laden air that has passed through the brushes is entrained downwardly by the air fed from above and, underneath the empty strand, is extracted together with the powder detached from the chain conveyor by the brushes and/or powder showered off by blowing in air.
A further preferred configuration of the invention makes provision, in the region of the interruption location or downline thereof in the direction of motion of the empty strand, for air not laden with powder to be fed into the housing from outside. This prevents a negative pressure build-up in the direction of motion of the empty strand downline of the interruption location, which in turn could. lead to powder-laden air being sucked in in this region. Alternatively or additionally, the air feed and, if appropriate, the air extraction in the region of the interruption location can also be set so that the quantity of air fed in is greater than the quantity of air extracted, in order to avoid the production of negative pressure.
In addition, by feeding the air not laden with powder, the air suction generated by the empty strand can be used for guiding this air past, if necessary together with the air blown in and deflected at the top of an extraction and air deflection device also serving as a dividing wall, above the dusting device in the direction of the printing machine. This air not laden with powder then forms an air cushion between the dusting device and the empty strand and between the dusting device and the printing machine, which likewise at the same time contributes to preventing the spread of the powder as far as the vicinity of the printing machine.
Furthermore, on the underside of the air cushion, air vortices are formed, at the underside of which fresh air flows back in the direction of the free end of the delivery. This in turn has the effect that, between the dusting device and the extraction device, some of the swirled powder is deflected to one or more extraction openings at the rear edge of the extraction device, whereat it can be extracted. On the other hand, between the dusting device and the printing machine, fresh air flows back on the underside of the vortex in the direction of the dusting device, as a result of which the swirled powder immediately above the printed sheets is forced into the direction towards the free end of the delivery. Surprisingly, more uniform dusting of both sides of the sheets can be achieved in this way.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for preventing uncontrolled spread of powder in a printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: